The great sperm race? Study reveals chaotic reproductive journey

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It isn't the most elegant of races.

In a study that attempts to elucidate one of the "central unsolved problems" in human reproduction — what separates the tens of sperm cells that find an egg, from the millions of cells ejaculated? — British researchers are reporting that sperm crawl, collide and crash head-on into the walls of the female reproductive tract in their frantic swim to the site of fertilization.

"Despite it being the thing that put all of us here, effectively, we still know almost nothing about how something so simple and key to life actually happens in the body," said Dr. Jackson Kirkman-Brown, a senior lecturer in reproductive sciences at the University of Birmingham in the U.K.

"Every time a woman gets or doesn't get pregnant, there is something going on where what to me is an incomprehensible number of sperm — two hundred million — get reduced to a countable amount, 10, maybe."

Popular wisdom holds that sperm follow each other in their swim through the female tract "rather like you or I might swim through a swimming pool," Kirkman-Brown said.

Instead, the researchers observed sperm travelling along walls, ceilings and particularly corners as they navigated the "narrow and convoluted channels" of the female tract, the team reports in the journal Proceedings of the National Academy of Sciences of the United States.

Sperm failing to get to the egg dooms fertility. "But equally some of the most common things in contraception are based on sperm motility," Kirkman-Brown said. Oral contraceptives — the Pill — work by thickening cervical mucous and stopping sperm from getting to the egg.

With ever more people undergoing fertility treatment and the pharmaceutical industry looking to develop new contraceptives, "There has been a lot more interest in sperm motility," he said.

To try to get a better handle on the "individual and group behaviours" that occur, the researchers watched sperm swimming not on slides or in petri dishes, but through tiny, hair-thin mini-mazes etched into silicone and barely visible to the naked eye.

The micro-channels were filled with fluid mimicking the viscosity of fluid within the female reproductive tract, giving sperm a more realistic environment. The mini mazes "have lots of different twists and turns, so we can see how sperm cope with swimming around these," Kirkman-Brown said.

Most people think of the uterus as an empty balloon. "But on the scale of a sperm, its actually very, very ridged," he said. There isn't one large empty central cavity waiting for a baby, he said. Rather, "it's moist surfaces that touch each other."

With the micro-channels, "we've given sperm some dimensions that are more like they would see in the body," he said. "That way we've been able to see how they may possibly — and I only say possibly — be behaving in the body, and learn a little bit more about how we can affect how they swim."

They found that when the channel turns sharply, cells leave the corner, continuing ahead until hitting the opposite wall of the channel.

"If you change the direction of the wall, if it's a very slight curve they will tend to still swim along that corner, and follow the corner," Kirkman-Brown said.

"But if you make a very sharp bend — so 90 degrees or more — they break away and will swim and hit the opposite wall, find the corner and then swim along that corner and follow the next corner that they find."

The researchers observed sperm colliding with walls and head butting into one another. "When they hit each other, they don't just hit, go past and carry on in the direction they were going," Kirkman-Brown explained. "Sometimes they'll stop, swerve a bit and then they swim off, both of them in different directions from where they were heading."

Similarly, when one sperm hits another from behind, he said, "they'll both splay out in new directions" rather like a bowling ball knocking down pins.

Lots of sperm fall by the wayside, and what the researchers are still some way from knowing is, what properties make the "best" swimmers?

The research potentially could lead to news ways of selecting only the best sperm for fertility treatments — in other words, the "Usain Bolt" of sperm, Kirkman-Brown said.

Most sperm a man makes are bad sperm, he said. The vast majority are abnormal in some way. "Having something that helps pick the best is quite interesting."

Kirman-Brown said he was surprised at how tenacious sperm are at swimming against walls and trying to follow corners, and how, when they touch each other it affects the direction in which they're swimming. "No one has previously been able to visualize how populations of sperm behave in micro-channels." Traditionally scientists have been restricted to counting sperm on microscopic slides, he said.

But sperm counts alone "isn't a very good indicator of whether you can get someone pregnant," he said.

He said sperm can swim four or five centimetres in an hour through the viscous fluids in the female tract.

On a human scale, "It's the equivalent, in an hour, of me getting halfway up Everest crawling in cold butter," he said. "The fact thåt something so tiny can do it under its own power is pretty incredible."

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